Metallic materials are known which contain 3 to 10% aluminum and 10 to 25% chromium, as well as one or more reactive elements of the row of silicon and/or zirconium and/or hafnium and/or titanium, in an amount less than 5%, and/or one or more of the rare earth elements in an amount less than 0.3%, and/or alkaline earth metals of the group Mg, Ba, Ca, Sr and Be in an amount between 0.001 and 1% as well as the trace elements normally present in steels, the remainder of the alloy being iron and/or nickel and/or cobalt. When the surface has been oxidized, the oxide layer produced on such alloys is designed to be rough so that it can also function in an advantageous manner as an adhesive base for further coatings, e.g. also for usage as catalytic carrier.
Metallic alloys of the type M Cr Al X and of the type M Cr Al Z X, in which M is iron and/or cobalt and/or nickel, and X represents small additives, weightwise, of highly reactive elements such as Y, Zr, Ti, Ce, Sm, Hf, La, Th, U, V, W, Ta, Nb, Mo, Gd, Si, Mg, Ca, and Z, which is present as an element or its oxide, is from the same row as X but which is an element different from that selected for X. These alloys have improved oxide layer properties (see Straford, K. N., "High Temperature Corrosion of Alloys Containing Rare Earth of Refractory Elements: A Review . . . ", High Temperature Technology, Vol. 1, No. 6, November 1983). In these alloys, the adhesion of the oxide layer is improved and thus the rate of oxidation is decreased.
It is also known that oxides of the rare earths such as Y.sub.2 O.sub.3, which are especially finely dispersed in a base alloy, exert a similar, improving influence (See Ramanarayan, T. A., Raghavan, M. and Petkovic-Luton, R., "The Characteristics of Alumina Scales Formed on Fe-Based Yttria-Dispersed Alloys", J. Electrochem. Society, April 1984, Vol. 131, No. 4, pp. 923-931).
Shell-shaped oxide can be produced in a known manner by special heat treatments on the surface of metallic materials from the latter. Thus, for example, published European Patent Application EP-A No. 009156 describes how whisker-shaped oxides can be produced from ferritic steels containing more than 0.002% of rare earths if they are exposed to a long-lasting oxidation in preferably dry air at approximately 900.degree. to 930.degree. C. A similar state of the art is also described in British Patent No. 2,063,723. The disadvantage of this technique resides in the necessity of having to add rare earths in order to increase the adhesive strength of the different types of oxide layers of the alloy of the metal. Rare earths are not only expensive but they also react in the course of the manufacturing process of the semi-finished product with oxygen, impurities and the crucible materials so that high losses arise.